Neuroradiology of posterior fossa tumors in children.

: In the examination of posterior fossa tumors, I believe that few findings are worthy of the term diagnostic. The small multiple focal calcifications of ependymomas are probably one such event, but it occurred in only 3 of the 124 tumors in this series. The large calcifications of medulloblastoma also seem to be close to a diagnostic sign, but this is also relatively uncommon. Extension to the midbrain or cerebellopontine angle in a large solid tumor is also fairly specific for medulloblastoma. A homogeneously decreased density before intravenous contrast seems to hold up as a sign of an astrocytoma in both this series and previously reported cases. Large cysts of the type typical for astrocytoma are quite uncommon in nonastrocytomas, but they do occur. Small cysts or cysts of any size in tumors that are less than 4 cm in total diameter are nonspecific findings, as are solid or cystic tumors whose solid portions are of mixed density. The relatively tumor-specific findings of the characteristic calcifications, low density and cysts of astrocytoma, and homogeneously hyperdense medulloblastomas account for approximately half of the tumors. Most of the remaining half can be accurately diagnosed by using combinations of size, location, and more general characteristics of density and contrast enhancement.